Conventional dry air vacuum cleaners are a very useful appliance for housekeeping to remove dirt or rubbish on a possible surface, for example, a floor, desk top or carpet. However, due to their structural limitation, conventional air vacuum cleaners cannot be used to extract liquid as liquid will possibly enter a motor chamber in the vacuum cleaner and cause damage to a motor therein. Such a limitation greatly lower the working abilities of the conventional dry air vacuum cleaners. In view of such a limitation, some vacuum cleaner manufacturers have developed vacuum cleaners which can have dual functions: dry suction and liquid extraction. Such dual functional vacuum cleaners have a totally different design from the conventional dry air vacuum cleaners: they must suitably separate the motor chamber from the dust collection chamber, arrange the two chambers in two different levels, and so on. Such a totally new design can indeed achieve the intended function to enable a vacuum cleaner to extract liquid; however, it must have a complicated structure which results in a somewhat unbearable high cost. Additionally, to buy a dual functional vacuum cleaner causes a waste for those who already have a dry air vacuum clear, since then their dry air vacuum cleaners become superfluous.